Method for device-to-device (d2d) operation performed by terminal in wireless communication system and terminal using the method

ABSTRACT

A method for device-to-device (D2D) operation performed by a terminal in a wireless communication system, and a terminal using the method are provided. The method comprises: receiving, from a network, D2D reception assistance information; and receiving a D2D signal by using a reception resource pool indicated by the D2D reception assistance information, wherein the D2D reception assistance information includes information that may reduce the range of resource pools which must be monitored by the terminal, among a plurality of resource pools.

BACKGROUND OF THE INVENTION Field of the invention

The present invention relates to wireless communications, and more particularly, relates to a method for a D2D operation performed by a terminal in a wireless communication system and a terminal using the method.

Related Art

In an International Telecommunication Union Radio communication sector (ITU-R), a standardization of International Mobile Telecommunication (IMT)-Advanced being a next mobile communication system after a third generation has been performed. The IMT-Advanced is aimed at supporting an Internet Protocol (IP) based multi-media service with a data transmission rate of 1 Gbps in a stop and low speed moving state and a data transmission rate of 1 Gbps in a high speed moving state.

A 3rd Generation Partnership Project (3GPP) is preparing LTE-Advanced (LTE-A) being an improved one of Long Term Evolution (LTE) based on an OFDMA(Orthogonal Frequency Division Multiple Access)/SC-FDMA(Single Carrier-Frequency Division Multiple Access) transmission scheme as a system standard satisfying requirements of IMT-Advanced. The LTE-A is one important candidate for IMT-Advanced.

In recent years, there is growing interest in a Device-to-Device (D2D) technology performing direct communication between devices. In particular, the D2D is attracting attention as a communication technology for a public safety network. A commercial communication network has been rapidly changed to the LTE but a current public safety network is based on a 2G technology in a collision problem and a cost side with an existing communication standard. Request for the technology clearance and an improved service induces an effort to improve the public safety network.

The public safety network has high service requirements (reliability and security) as compared with a commercial communication network. In particular, when coverage of cellular communication is insufficient or is not used, there is a need for direct signal transmission/reception between devices, that is, an D2D operation.

The D2D operation may be signal transmission/reception between adjacent devices to have various advantages. For example, a D2D terminal may perform data communication with a high transmission rate and low delay. Further, the D2D operation may distribute traffic converged in a base station. If the D2D terminal serves as a relay, the D2D terminal may serve to extend coverage of a base station.

Meanwhile, a network notifies a terminal the resources that may be used for the D2D operation. A method is required for configuring the signal that notifies such resources. In addition, a terminal should monitor a plurality of resource pools for receiving the D2D signal. In this case, the power of terminal may be wasted if the terminal should also monitor an unnecessary resource pool. Accordingly, a method is required to notify the resource that may be used for the D2D operation efficiently.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for a D2D operation performed by a terminal in a wireless communication system and a terminal using the method.

In an aspect, a method for a device-to-device (D2D) operation performed by a user equipment (UE) in a wireless communication system is provided, the method comprises receiving, from a network, D2D reception support information and receiving a D2D signal using a reception resource pool indicated by the D2D reception support information, wherein the D2D reception support information includes information that is available to decrease a range of resource pool that the UE should monitor among a plurality of resource pools.

The method may further comprises transmitting a D2D reception support information request to the network.

The request may include an identity (ID) or a group ID of another UE that transmits the D2D signal that the UE is going to receive.

The request may include identification information that is available to identify the UE.

The D2D reception support information may notify a reference cell that the UE should monitor.

The D2D reception support information may indicate a specific resource pool among the plurality of resource pools.

The method may further comprise receiving resource pool information, wherein the resource pool information indicates the plurality of resource pools.

In another aspect, a user equipment (UE) for performing a device-to-device (D2D) operation in a wireless communication system is provided. The UE comprises a radio frequency (RF) unit that transmits or receives a radio signal and a processor operatively connected to the RF unit, wherein the processor: receives D2D reception support information from a network and receives a D2D signal using a reception resource pool indicated by the D2D reception support information, wherein the D2D reception support information includes information that is available to decrease a range of resource pool that the UE should monitor among a plurality of resource pools.

ADVANTAGEOUS EFFECTS

According to the present invention, the amount/range of resource that a terminal should monitor for receiving the D2D signal may be decreased. That is, the optimization of D2D signal monitoring of UE is available. Using the method, the power consumption of terminal may be decreased. In addition, since there are more chances for a terminal to monitor the resource that is indispensible for receiving the D2D signal, the probability of receiving the D2D signal successfully may also be increased. Accordingly, the reliability of the D2D operation is increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the present invention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a user plane.

FIG. 3 is a diagram showing a wireless protocol architecture for a control plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idle state.

FIG. 5 is a flowchart illustrating a procedure of establishing RRC connection.

FIG. 6 is a flowchart illustrating an RRC connection reconfiguration procedure.

FIG. 7 is a diagram illustrating an RRC connection re-establishment procedure.

FIG. 8 illustrates sub states where the terminal may have in an RRC IDLE state and a sub state transition process.

FIG. 9 illustrates a reference structure for a ProSe.

FIG. 10 illustrates arrangement examples of terminals performing ProSe direct communication and cell coverage.

FIG. 11 illustrates a user plane protocol stack for the ProSe direct communication.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

FIG. 13 illustrates an embodiment of a ProSe direct discovery procedure.

FIG. 14 illustrates another embodiment of a ProSe direct discovery procedure.

FIG. 15 exemplifies the case to which the present invention may be applied.

FIG. 16 exemplifies a D2D operation of UE according to the first method.

FIG. 17 illustrates a D2D operation method of UE according to an embodiment of the present invention.

FIG. 18 illustrates a D2D operation method of UE according to an embodiment of the present invention.

FIG. 19 is a block diagram illustrating a UE in which the embodiments of the present invention are implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the present invention is applied. The wireless communication system may also be referred to as an evolved-UMTS terrestrial radio access network (E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides a control plane and a user plane to a user equipment (UE) 10. The UE 10 may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a mobile terminal (MT), a wireless device, etc. The BS 20 is generally a fixed station that communicates with the UE 10 and may be referred to as another terminology, such as an evolved node-B (eNB), a base transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20 are also connected by means of an S1 interface to an evolved packet core (EPC) 30, more specifically, to a mobility management entity (MME) through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway (P-GW). The MME has access information of the UE or capability information of the UE, and such information is generally used for mobility management of the UE. The S-GW is a gateway having an E-UTRAN as an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network can be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on the lower three layers of the open system interconnection (OSI) model that is well-known in the communication system. Among them, a physical (PHY) layer belonging to the first layer provides an information transfer service by using a physical channel, and a radio resource control (RRC) layer belonging to the third layer serves to control a radio resource between the UE and the network. For this, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a user plane. FIG. 3 is a diagram showing a wireless protocol architecture for a control plane. The user plane is a protocol stack for user data transmission. The control plane is a protocol stack for control signal transmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with an information transfer service through a physical channel. The PHY layer is connected to a medium access control (MAC) layer which is an upper layer of the PHY layer through a transport channel. Data is transferred between the MAC layer and the PHY layer through the transport channel. The transport channel is classified according to how and with what characteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of a transmitter and a receiver, through a physical channel. The physical channel may be modulated according to an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and use the time and frequency as radio resources.

The functions of the MAC layer include mapping between a logical channel and a transport channel and multiplexing and demultiplexing to a transport block that is provided through a physical channel on the transport channel of a MAC Service Data Unit (SDU) that belongs to a logical channel. The MAC layer provides service to a Radio Link Control (RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation, and reassembly of an RLC SDU. In order to guarantee various types of Quality of Service (QoS) required by a Radio Bearer (RB), the RLC layer provides three types of operation mode: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provides error correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer is related to the configuration, reconfiguration, and release of radio bearers, and is responsible for control of logical channels, transport channels, and PHY channels. An RB means a logical route that is provided by the first layer (PHY layer) and the second layers (MAC layer, the RLC layer, and the PDCP layer) in order to transfer data between UE and a network.

The function of a Packet Data Convergence Protocol (PDCP) layer on the user plane includes the transfer of user data and header compression and ciphering. The function of the PDCP layer on the user plane further includes the transfer and encryption/integrity protection of control plane data.

What an RB is configured means a procedure of defining the characteristics of a wireless protocol layer and channels in order to provide specific service and configuring each detailed parameter and operating method. An RB can be divided into two types of a Signaling RB (SRB) and a Data RB (DRB). The SRB is used as a passage through which an RRC message is transmitted on the control plane, and the DRB is used as a passage through which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRC layer of an E-UTRAN, the UE is in the RRC connected state. If not, the UE is in the RRC idle state.

A downlink transport channel through which data is transmitted from a network to UE includes a broadcast channel (BCH) through which system information is transmitted and a downlink shared channel (SCH) through which user traffic or control messages are transmitted. Traffic or a control message for downlink multicast or broadcast service may be transmitted through the downlink SCH, or may be transmitted through an additional downlink multicast channel (MCH). Meanwhile, an uplink transport channel through which data is transmitted from UE to a network includes a random access channel (RACH) through which an initial control message is transmitted and an uplink shared channel (SCH) through which user traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that are mapped to the transport channel include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic channel (MTCH).

The physical channel includes several OFDM symbols in the time domain and several subcarriers in the frequency domain. One subframe includes a plurality of OFDM symbols in the time domain. An RB is a resources allocation unit, and includes a plurality of OFDM symbols and a plurality of subcarriers. Furthermore, each subframe may use specific subcarriers of specific OFDM symbols (e.g., the first OFDM symbol) of the corresponding subframe for a physical downlink control channel (PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval (TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logically connected to the RRC layer of the E-UTRAN. A case where the RRC layer of UE is logically connected to the RRC layer of the E-UTRAN is referred to as an RRC connected state. A case where the RRC layer of UE is not logically connected to the RRC layer of the E-UTRAN is referred to as an RRC idle state. The E-UTRAN may check the existence of corresponding UE in the RRC connected state in each cell because the UE has RRC connection, so the UE may be effectively controlled. In contrast, the E-UTRAN is unable to check UE in the RRC idle state, and a Core Network (CN) manages UE in the RRC idle state in each tracking area, that is, the unit of an area greater than a cell. That is, the existence or non-existence of UE in the RRC idle state is checked only for each large area. Accordingly, the UE needs to shift to the RRC connected state in order to be provided with common mobile communication service, such as voice or data.

When a user first powers UE, the UE first searches for a proper cell and remains in the RRC idle state in the corresponding cell. The UE in the RRC idle state establishes RRC connection with an E-UTRAN through an RRC connection procedure when it is necessary to set up the RRC connection, and shifts to the RRC connected state. A case where UE in the RRC idle state needs to set up RRC connection includes several cases. For example, the cases may include a need to send uplink data for a reason, such as a call attempt by a user, and to send a response message as a response to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performs functions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types of states: EPS Mobility Management-REGISTERED (EMM-REGISTERED) and EMM-DEREGISTERED are defined. The two states are applied to UE and the MME. UE is initially in the EMM-DEREGISTERED state. In order to access a network, the UE performs a procedure of registering it with the corresponding network through an initial attach procedure. If the attach procedure is successfully performed, the UE and the MME become the EMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, two types of states: an EPS Connection Management (ECM)-IDLE state and an ECM-CONNECTED state are defined. The two states are applied to UE and the MME. When the UE in the ECM-IDLE state establishes RRC connection with the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in the ECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1 connection with the E-UTRAN. When the UE is in the ECM-IDLE state, the E-UTRAN does not have information about the context of the UE. Accordingly, the UE in the ECM-IDLE state performs procedures related to UE-based mobility, such as cell selection or cell reselection, without a need to receive a command from a network. In contrast, when the UE is in the ECM-CONNECTED state, the mobility of the UE is managed in response to a command from a network. If the location of the UE in the ECM-IDLE state is different from a location known to the network, the UE informs the network of its corresponding location through a tracking area update procedure.

System information is described below.

System information includes essential information that needs to be known by UE in order for the UE to access a BS. Accordingly, the UE needs to have received all pieces of system information before accessing the BS, and needs to always have the up-to-date system information. Furthermore, the BS periodically transmits the system information because the system information is information that needs to be known by all UEs within one cell. The system information is divided into a Master Information Block (MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include a limited number of parameters that are most essential and most frequently transmitted when other information is required to be obtained from a cell. UE first searches for an MIB after downlink synchronization. The MIB may include information, such as an SFN that supports downlink channel bandwidth, a PHICH configuration, and synchronization and operates as a timing criterion and an eNB transmit antenna configuration. The MIB may be transmitted on a broadcast channel (BCH) through broadcasting.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a “SystemInformationBlockType1” message and transmitted. The remaining SIBs other than the SIB1 is included in a system information message and transmitted. To map the SIBs to the system information message may be flexibly configured by a scheduling information list parameter included in the SIB1. In this case, each of the SIBs is included in a single system information message, and only SIBs having the same scheduling requirement value (e.g. cycle) may be mapped to the same system information message. Furthermore, a SystemInformationBlockType2 (SIB2) is always mapped to a system information message corresponding to the first entry within the system information message list of a scheduling information list. A plurality of system information messages may be transmitted within the same cycle. The SIB1 and all the system information messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in an E-UTRAN, the SIB1 may be dedicated-signaled in the state in which it includes a parameter configured like an existing configured value. In this case, the SIB1 may be included in an RRC connection reconfiguration message and transmitted.

The SIB1 includes information related to UE cell access, and defines the scheduling of other SIBs. The SIB1 may include information related to the PLMN identifiers of a network, tracking area code (TAC) and a cell ID, a cell barring status indicative of whether a cell is a cell on which camp-on is possible, the lowest reception level required within a cell which is used as cell reselection criterion, and the transmission time and cycle of other SIBs.

The SIB2 may include radio resource configuration information common to all pieces of UE. The SIB2 may include information related to an uplink carrier frequency and uplink channel bandwidth, an RACH configuration, a page configuration, an uplink power control configuration, a sounding reference signal configuration, a PUCCH configuration supporting ACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and detecting a change of system information to a primary cell (PCell) only. In a secondary cell (SCell), when a corresponding SCell is added, an E-UTRAN may provide all of pieces of system information related to an RRC connection state operation through dedicated signaling. When system information related to a configured SCell is changed, an E-UTRAN may release an SCell that is taken into consideration and subsequently add the changed system information. This may be performed along with a single RRC connection reconfiguration message. An E-UTRAN may configure parameter values different from a value broadcasted within an SCell that has been taken into consideration through dedicated signaling.

UE needs to guarantee the validity of a specific type of system information, and such system information is called required system information. The required system information may be defined as follows.

If UE is an RRC idle state: The UE needs to be guaranteed so that it has the valid versions of the MIB and the SIB1 in addition to the SIB2 to SIB8. This may comply with the support of a radio access technology (RAT) that is taken into consideration.

If UE is an RRC connection state: The UE needs to be guaranteed so that it has the valid versions of the MIB, the SIB1, and the SIB2.

In general, the validity of system information may be guaranteed up to a maximum of 3 hours after the system information is obtained.

In general, service that is provided to UE by a network may be classified into three types as follows. Furthermore, the UE differently recognizes the type of cell depending on what service may be provided to the UE. In the following description, a service type is first described, and the type of cell is described.

1) Limited service: this service provides emergency calls and an Earthquake and Tsunami Warning System (ETWS), and may be provided by an acceptable cell.

2) Suitable service: this service means public service for common uses, and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communication network operators. This cell may be used by only communication network operators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell may be classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be provided with limited service. This cell is a cell that has not been barred from a viewpoint of corresponding UE and that satisfies the cell selection criterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be provided with suitable service. This cell satisfies the conditions of an acceptable cell and also satisfies additional conditions. The additional conditions include that the suitable cell needs to belong to a Public Land Mobile Network (PLMN) to which corresponding UE may access and that the suitable cell is a cell on which the execution of a tracking area update procedure by the UE is not barred. If a corresponding cell is a CSG cell, the cell needs to be a cell to which UE may access as a member of the CSG.

3) A barred cell: this cell is a cell that broadcasts information indicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts information indicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idle state. FIG. 4 illustrates a procedure in which UE that is initially powered on experiences a cell selection procedure, registers it with a network, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) in which the UE communicates with a Public Land Mobile Network (PLMN), that is, a network from which the UE is provided with service (S410). Information about the PLMN and the RAT may be selected by the user of the UE, and the information stored in a Universal Subscriber Identity Module (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs to cells having measured BS and signal intensity or quality greater than a specific value (cell selection) (S420). In this case, the UE that is powered off performs cell selection, which may be called initial cell selection. A cell selection procedure is described later in detail. After the cell selection, the UE receives system information periodically by the BS. The specific value refers to a value that is defined in a system in order for the quality of a physical signal in data transmission/reception to be guaranteed. Accordingly, the specific value may differ depending on applied RAT.

If network registration is necessary, the UE performs a network registration procedure (S430). The UE registers its information (e.g., an IMSI) with the network in order to receive service (e.g., paging) from the network. The UE does not register it with a network whenever it selects a cell, but registers it with a network when information about the network (e.g., a Tracking Area Identity (TAI)) included in system information is different from information about the network that is known to the UE.

The UE performs cell reselection based on a service environment provided by the cell or the environment of the UE (S440). If the value of the intensity or quality of a signal measured based on a BS from which the UE is provided with service is lower than that measured based on a BS of a neighboring cell, the UE selects a cell that belongs to other cells and that provides better signal characteristics than the cell of the BS that is accessed by the UE. This procedure is called cell reselection differently from the initial cell selection of the No. 2 procedure. In this case, temporal restriction conditions are placed in order for a cell to be frequently reselected in response to a change of signal characteristic. A cell reselection procedure is described later in detail.

FIG. 5 is a flowchart illustrating a procedure of establishing RRC connection.

UE sends an RRC connection request message that requests RRC connection to a network (S510). The network sends an RRC connection establishment message as a response to the RRC connection request (S520). After receiving the RRC connection establishment message, the UE enters RRC connected mode.

The UE sends an RRC connection establishment complete message used to check the successful completion of the RRC connection to the network (S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfiguration procedure. An RRC connection reconfiguration is used to modify RRC connection. This is used to establish/modify/release RBs, perform handover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifying RRC connection to UE (S610). As a response to the RRC connection reconfiguration message, the UE sends an RRC connection reconfiguration complete message used to check the successful completion of the RRC connection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile network operator. Each mobile network operator operates one or more PLMNs. Each PLMN may be identified by a Mobile Country Code (MCC) and a Mobile Network Code (MNC). PLMN information of a cell is included in system information and broadcasted.

In PLMN selection, cell selection, and cell reselection, various types of PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC of a terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing location registration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a general service is provided to the terminal through the HPLMN or the EHPLMN, the terminal is not in a roaming state. Meanwhile, when the service is provided to the terminal through a PLMN except for the HPLMN/EHPLMN, the terminal is in the roaming state. In this case, the PLMN refers to a Visited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available Public Land Mobile Networks (PLMNs) and selects a proper PLMN from which the UE is able to be provided with service. The PLMN is a network that is deployed or operated by a mobile network operator. Each mobile network operator operates one or more PLMNs. Each PLMN may be identified by Mobile Country Code (MCC) and Mobile Network Code (MNC). Information about the PLMN of a cell is included in system information and broadcasted. The UE attempts to register it with the selected PLMN. If registration is successful, the selected PLMN becomes a Registered PLMN (RPLMN). The network may signalize a PLMN list to the UE. In this case, PLMNs included in the PLMN list may be considered to be PLMNs, such as RPLMNs. The UE registered with the network needs to be able to be always reachable by the network. If the UE is in the ECM-CONNECTED state (identically the RRC connection state), the network recognizes that the UE is being provided with service. If the UE is in the ECM-IDLE state (identically the RRC idle state), however, the situation of the UE is not valid in an eNB, but is stored in the MME. In such a case, only the MME is informed of the location of the UE in the ECM-IDLE state through the granularity of the list of Tracking Areas (TAs). A single TA is identified by a Tracking Area Identity (TAI) formed of the identifier of a PLMN to which the TA belongs and Tracking Area Code (TAC) that uniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by the selected PLMN and that has signal quality and characteristics on which the UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting a cell by a terminal.

When power is turned-on or the terminal is located in a cell, the terminal performs procedures for receiving a service by selecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a service through the cell by always selecting a suitable quality cell. For example, a terminal where power is turned-on just before should select the suitable quality cell to be registered in a network. If the terminal in an RRC connection state enters in an RRC idle state, the terminal should selects a cell for stay in the RRC idle state. In this way, a procedure of selecting a cell satisfying a certain condition by the terminal in order to be in a service idle state such as the RRC idle state refers to cell selection. Since the cell selection is performed in a state that a cell in the RRC idle state is not currently determined, it is important to select the cell as rapid as possible. Accordingly, if the cell provides a wireless signal quality of a predetermined level or greater, although the cell does not provide the best wireless signal quality, the cell may be selected during a cell selection procedure of the terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTE is described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “User Equipment (UE) procedures in idle mode (Release 8)”.

A cell selection procedure is basically divided into two types.

The first is an initial cell selection procedure. In this procedure, UE does not have preliminary information about a wireless channel. Accordingly, the UE searches for all wireless channels in order to find out a proper cell. The UE searches for the strongest cell in each channel. Thereafter, if the UE has only to search for a suitable cell that satisfies a cell selection criterion, the UE selects the corresponding cell.

Next, the UE may select the cell using stored information or using information broadcasted by the cell. Accordingly, cell selection may be fast compared to an initial cell selection procedure. If the UE has only to search for a cell that satisfies the cell selection criterion, the UE selects the corresponding cell. If a suitable cell that satisfies the cell selection criterion is not retrieved though such a procedure, the UE performs an initial cell selection procedure.

A cell selection criterion may be defined as in Equation 1 below.

Srxlev>0 AND Squal>0,   [Equation 1]

where:

Srxlev=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(rxlevminoffset))−P _(compensation),

Squal=Q _(qualmeas)−(Q _(qualmin) +Q _(qualminoffset))

In this case, in Equation 1, the variables may be defined as in Table 1 below.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selection quality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP) Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimum required RX level in the cell (dBm) Q_(qualmin) Minimum required quality level in the cell (dB) Q_(rxlevminoffset) Offset to the signalled Q_(rxlevmin) taken into account in the Srxlev evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken into account in the Squal evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN Pcompensation max(P_(EMAX) − P_(PowerClass), 0) (dB) P_(EMAX) Maximum TX power level an UE may use when transmitting on the uplink in the cell (dBm) defined as P_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of the UE (dBm) according to the UE power class as defined in [TS 36.101]

Qrxlevminoffset and Qqualminoffset, that is, signaled values, are the results of periodic discovery for a PLMN having higher priority while UE camps on a normal cell within a VPLMN, and may be applied only when cell selection is evaluated. As described above, during the periodic discovery of a PLMN having higher priority, UE may perform cell selection evaluation using parameter values stored from another cell of the PLMN having such higher priority.

After UE selects any cell through a cell selection procedure, the intensity or quality of a signal between the UE and a BS may be changed due to the mobility of the UE or a change of a radio environment. Accordingly, if the quality of the selected cell is changed, the UE may select another cell providing better quality.

After the UE selects a specific cell through the cell selection procedure, the intensity or quality of a signal between the UE and a BS may be changed due to a change in the mobility or wireless environment of the UE. Accordingly, if the quality of the selected cell is deteriorated, the UE may select another cell that provides better quality. If a cell is reselected as described above, the UE selects a cell that provides better signal quality than the currently selected cell. Such a procedure is called cell reselection. In general, a basic object of the cell reselection procedure is to select a cell that provides UE with the best quality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a network may determine priority corresponding to each frequency, and may inform the UE of the determined priorities. The UE that has received the priorities preferentially takes into consideration the priorities in a cell reselection procedure compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cell according to the signal characteristics of a wireless environment. In selecting a cell for reselection when a cell is reselected, the following cell reselection methods may be present according to the RAT and frequency characteristics of the cell.

Intra-frequency cell reselection: UE reselects a cell having the same center frequency as that of RAT, such as a cell on which the UE camps on.

Inter-frequency cell reselection: UE reselects a cell having a different center frequency from that of RAT, such as a cell on which the UE camps on

Inter-RAT cell reselection: UE reselects a cell that uses RAT different from RAT on which the UE camps

The principle of a cell reselection procedure is as follows.

First, UE measures the quality of a serving cell and neighbor cells for cell reselection.

Second, cell reselection is performed based on a cell reselection criterion. The cell reselection criterion has the following characteristics in relation to the measurements of a serving cell and neighbor cells.

Intra-frequency cell reselection is basically based on ranking. Ranking is a task for defining a criterion value for evaluating cell reselection and numbering cells using criterion values according to the size of the criterion values. A cell having the best criterion is commonly called the best-ranked cell. The cell criterion value is based on the value of a corresponding cell measured by UE, and may be a value to which a frequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority provided by a network. UE attempts to camp on a frequency having the highest frequency priority. A network may provide frequency priority that will be applied by UEs within a cell in common through broadcasting signaling, or may provide frequency-specific priority to each UE through UE-dedicated signaling. A cell reselection priority provided through broadcast signaling may refer to a common priority. A cell reselection priority for each terminal set by a network may refer to a dedicated priority. If receiving the dedicated priority, the terminal may receive a valid time associated with the dedicated priority together. If receiving the dedicated priority, the terminal starts a validity timer set as the received valid time together therewith. While the valid timer is operated, the terminal applies the dedicated priority in the RRC idle mode. If the valid timer is expired, the terminal discards the dedicated priority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE with a parameter (e.g., a frequency-specific offset) used in cell reselection for each frequency. For the intra-frequency cell reselection or the inter-frequency cell reselection, a network may provide UE with a Neighboring Cell List (NCL) used in cell reselection. The NCL includes a cell-specific parameter (e.g., a cell-specific offset) used in cell reselection. For the intra-frequency or inter-frequency cell reselection, a network may provide UE with a cell reselection black list used in cell reselection.

The UE does not perform cell reselection on a cell included in the black list.

Ranking performed in a cell reselection evaluation procedure is described below.

A ranking criterion used to give the priority of a cell is defined as in Equation 2.

R _(s) =Q _(meas,s) +Q _(hyst) , R _(n) =Q _(meas,n) −Q _(offset)   [Equation 1]

In Equation 2, Rs is the ranking criterion of a serving cell on which UE now camps, Rn is the ranking criterion of a neighboring cell, Qmeas,s is the quality value of the serving cell measured by the UE, Qmeas,n is the quality value of the neighboring cell measured by the UE, Qhyst is a hysteresis value for ranking, and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between a serving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does not Qoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for a corresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does not receive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterion Rn of a neighbor cell are changed in a similar state, ranking priority is frequency changed as a result of the change, and UE may alternately reselect the twos. Qhyst is a parameter that gives hysteresis to cell reselection so that UE is prevented from to alternately reselecting two cells.

UE measures RS of a serving cell and Rn of a neighbor cell according to the above equation, considers a cell having the greatest ranking criterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality of a cell is the most important criterion in cell reselection. If a reselected cell is not a suitable cell, UE excludes a corresponding frequency or a corresponding cell from the subject of cell reselection.

Hereinafter, radio link failure (RLF) will be described.

UE continues to perform measurements in order to maintain the quality of a radio link with a serving cell from which the UE receives service. The UE determines whether or not communication is impossible in a current situation due to the deterioration of the quality of the radio link with the serving cell. If communication is almost impossible because the quality of the serving cell is too low, the UE determines the current situation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication with the current serving cell, selects a new cell through cell selection (or cell reselection) procedure, and attempts RRC connection re-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken as cases where normal communication is impossible.

A case where UE determines that there is a serious problem in the quality of a downlink communication link (a case where the quality of a PCell is determined to be low while performing RLM) based on the radio quality measured results of the PHY layer of the UE

A case where uplink transmission is problematic because a random access procedure continues to fail in the MAC sublayer.

A case where uplink transmission is problematic because uplink data transmission continues to fail in the RLC sublayer.

A case where handover is determined to have failed.

A case where a message received by UE does not pass through an integrity check.

An RRC connection re-establishment procedure is described in more detail below.

FIG. 7 is a diagram illustrating an RRC connection re-establishment procedure.

Referring to FIG. 7, UE stops using all the radio bearers that have been configured other than a Signaling Radio Bearer (SRB) #0, and initializes a variety of kinds of sublayers of an Access Stratum (AS) (S710). Furthermore, the UE configures each sublayer and the PHY layer as a default configuration. In this procedure, the UE maintains the RRC connection state.

The UE performs a cell selection procedure for performing an RRC connection reconfiguration procedure (S720). The cell selection procedure of the RRC connection re-establishment procedure may be performed in the same manner as the cell selection procedure that is performed by the UE in the RRC idle state, although the UE maintains the RRC connection state.

After performing the cell selection procedure, the UE determines whether or not a corresponding cell is a suitable cell by checking the system information of the corresponding cell (S730). If the selected cell is determined to be a suitable E-UTRAN cell, the UE sends an RRC connection re-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RAT different from that of the E-UTRAN through the cell selection procedure for performing the RRC connection re-establishment procedure, the UE stops the RRC connection re-establishment procedure and enters the RRC idle state (S750).

The UE may be implemented to finish checking whether the selected cell is a suitable cell through the cell selection procedure and the reception of the system information of the selected cell. To this end, the UE may drive a timer when the RRC connection re-establishment procedure is started. The timer may be stopped if it is determined that the UE has selected a suitable cell. If the timer expires, the UE may consider that the RRC connection re-establishment procedure has failed, and may enter the RRC idle state. Such a timer is hereinafter called an RLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as an RLF timer. The UE may obtain the set value of the timer from the system information of the serving cell.

If an RRC connection re-establishment request message is received from the UE and the request is accepted, a cell sends an RRC connection re-establishment message to the UE.

The UE that has received the RRC connection re-establishment message from the cell reconfigures a PDCP sublayer and an RLC sublayer with an SRB1. Furthermore, the UE calculates various key values related to security setting, and reconfigures a PDCP sublayer responsible for security as the newly calculated security key values. Accordingly, the SRB 1 between the UE and the cell is open, and the UE and the cell may exchange RRC control messages. The UE completes the restart of the SRB1, and sends an RRC connection re-establishment complete message indicative of that the RRC connection re-establishment procedure has been completed to the cell (S760).

In contrast, if the RRC connection re-establishment request message is received from the UE and the request is not accepted, the cell sends an RRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfully performed, the cell and the UE perform an RRC connection reconfiguration procedure. Accordingly, the UE recovers the state prior to the execution of the RRC connection re-establishment procedure, and the continuity of service is guaranteed to the upmost.

FIG. 8 illustrates sub states where the terminal may have in an RRC IDLE state and a sub state transition process.

Referring to FIG. 8, a terminal performs an initial cell selection process (S801). The initial cell selection process may be performed when there is no stored cell information with respect to the PLMN or a suitable cell is not found.

If the suitable cell is not found in the initial cell selection process, the terminal transitions to an any cell selection state (S802). The optional cell selection state represents a state which does not camp on in both of a suitable cell and an acceptable cell. The optional cell selection state is a state attempted by the terminal in order to find an acceptable cell of an optional PLMN which may camp on. When the terminal finds no cells which may camp on, the terminal is continuously maintained in an optional cell selection state until the acceptable cell is found.

If the suitable cell is found in the initial cell selection process, the state transits to a normal camp state (S803). The normal camp state represents a state which camps on the normal cell. A paging channel is selected according to information given through system information to motor, and an evaluation process for cell reselection may be performed.

In the normal camp state (S803), if a cell reselection evaluation process (S804) is caused, the cell reselection evaluation process (S804) is performed. If a suitable cell is found in the cell reselection evaluation process (S804), the terminal again transits to the normal camp state (S803).

If an acceptable cell is found in the any cell selection state (S802), the terminal transits to an any cell camped state (S805). The any cell camped state (S805) represents a state of camping on an acceptable cell.

In the any cell camped state (S805), the terminal may select a paging channel according to information given through system information to monitor, and may perform a cell reselection evaluation process (S806). If the acceptable cell is not found in the cell reselection evaluation process (S806), the terminal transits the any cell selection state (S802).

Hereinafter, a D2D operation will be described. In the 3GPP LTE-A, a service related to the D2D operation refers to Proximity based Services (ProSe). Hereinafter, the ProSe is an equivalent concept with the D2D operation and the ProSe may be compatibly used with the D2D operation. The ProSe is now described.

The ProSe includes ProSe direct communication and ProSe direct discovery. The ProSe direct communication presents communication performed by two or more adjacent terminals. The terminals may perform communication using a protocol of a user plane. A ProSe-enabled UE means a UE for supporting a process related to requirements of the ProSe. Unless otherwise defined, the ProSe-enabled UE includes both of a public safety UE and a non-public safety UE. The public safety UE represents a UE for supporting both of a public safety specified function and the ProSe process. The non-public safety UE is a terminal which supports the ProSe process but does not support the public safety specified function.

The ProSe direct discovery is a process where the ProSe-enabled UE discovers another ProSe-enabled UE. In this case, only ability of the two ProSe-enabled UEs is used. An EPC-level ProSe discovery signifies a process where an EPC determines whether 2 ProSe enable terminals are closed to each other, and reports the close state thereof the two ProSe enabled terminals.

Hereinafter, the ProSe direct communication may refer to D2D communication, and the ProSe direct discovery may refer to D2D discovery.

FIG. 9 illustrates a reference structure for a ProSe.

Referring to FIG. 9, the reference structure for a ProSe includes a plurality of terminals having E-UTRAN, EPC, and ProSe application program, a ProSe application (APP) server, and a ProSe function.

An EPC is a representative example of the E-UTRAN. The EPC may include an MME, an S-GW, a P-GW, a policy and charging rules function (PCRF), and a home subscriber server (HSS).

The ProSe application server is a user of ProSe in order to make an application function. The ProSe application server may communicate with an application program in the terminal. The application program in the terminal may use a ProSe ability to make an application function.

The ProSe function may include at least one of following functions but is not limited thereto.

Interworking via a reference point towards the 3rd party applications

Authorization and configuration of the UE for discovery and direct communication)

Enable the function of the EPC level ProSe discovery

ProSe related new subscriber data and handling of data storage, and also handling of

ProSe identities

Security related function

Provide control towards the EPC for policy related function

Provide function for charging (via or outside of EPC, e.g., offline charging))

Hereinafter, a reference point and a reference interface will be described in a reference structure for the ProSe.

PC1: a reference point between a ProSe application program in the terminal and a ProSe application program in a ProSe application server. The PC1 is used to define signaling requirements in an application level.

PC2: is a reference point between the ProSe application server and a ProSe function. The PC2 is used to define an interaction between the ProSe application server and a ProSe function. An application data update of a ProSe database of the ProSe function may be an example of the interaction.

PC3: is a reference point between the terminal and the ProSe function. The PC3 is used to define an interaction between the terminal and the ProSe function. Configuration for ProSe discovery and communication may be an example of the interaction.

PC4: is a reference point between an EPC and the ProSe function. The PC4 is used to define an interaction between the EPC and the ProSe function. The interaction lay illustrate when a path for 1:1 communication or a ProSe service for real time session management or mobility management are authorized.

PC5: is a reference point to use control/user plane for discovery, communication, and relay between terminals, and 1:1 communication.

PC6: is a reference point to use a function such as ProSe discovery between users included in different PLMNs.

SGi: may be used for application data and application level control information exchange.

<ProSe Direct Communication (D2D Communication)>.

The ProSe direct communication is a communication mode where two public safety terminals may perform direct communication through a PC 5 interface. The communication mode may be supported in both of a case of receiving a service in coverage of E-UTRAN or a case of separating the coverage of E-UTRAN.

FIG. 10 illustrates arrangement examples of terminals performing ProSe direct communication and cell coverage.

Referring to FIG. 10(a), UEs A and B may be located outside of the cell coverage. Referring to FIG. 10(b), the UE A may be located in the cell coverage and the UE B may be located outside of the cell coverage. Referring to FIG. 10(c), both of UEs A and B may be located in the cell coverage. Referring to FIG. 10(d), the UE A may be located in coverage of a first cell and the UE B may be in coverage of a second cell.

As described above, the ProSe direct communication may be performed between terminals which are provided at various positions.

Meanwhile, following IDs may be used in the ProSe direct communication.

Source layer-2 ID: The source layer-2 ID identifies a sender of a packet in a PC 5 interface.

Purpose layer-2 ID: The purpose layer-2 ID identifies a target of a packet in a PC 5 interface.

SA L1 ID: The SA L1 ID represents an in an ID in a scheduling assignment (SA) in the PC 5 interface.

FIG. 11 illustrates a user plane protocol stack for the ProSe direct communication.

Referring to FIG. 11, the PC 5 interface includes a PDCH layer, a RLC layer, a MAC layer, and a PHY layer.

There may not be HARQ feedback in the ProSe direct communication. An MAC header may include the source layer-2 ID and the purpose layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>.

A ProSe enable terminal may use following two modes with respect to resource assignments for the ProSe direct communication.

1. Mode 1

The mode 2 is a mode for receiving scheduling a resource for the ProSe direct communication from a base station. The terminal should be in a RRC_CONNECTED state according to the mode 1 in order to transmit data. The terminal requests a transmission resource to the base station, and the base station schedules a resource for scheduling assignment and data transmission. The terminal may transmit a scheduling request to the base station and may transmit a Buffer Status Report (ProSe BSR). The base station has data which the terminal will perform the ProSe direct communication and determines whether a resource for transmitting the data is required.

2. Mode 2

The mode 2 is a mode for selecting a direct resource. The terminal directly selects a resource for the ProSe direct communication from a resource pool. The resource pool may be configured by a network or may be previously determined.

Meanwhile, when the terminal includes a serving cell, that is, when the terminal is in an RRC_CONNECTED state with the base station or is located in a specific cell in an RRC_IDLE state, the terminal is regarded to be in coverage of the base station.

If the terminal is located outside of the coverage, only the mode 2 is applicable. If the terminal is located in the coverage, the mode 1 or the mode 2 may be used according to setting of the base station.

If there are no exceptional conditions, only when the base station is configured, the terminal may change a mode from the mode 1 to the mode 2 or from the mode 2 to the mode 1.

<ProSe Direct Discovery (D2D Discovery)>

The ProSe direct discovery represents a process used to discover when the ProSe enabled terminal discovers other neighboring ProSe enabled terminal and refers to D2D direction discovery or D2D discovery. In this case, an E-UTRA wireless signal through the PC 4 interface may be used. Hereinafter, information used for the ProSe direct discovery refers to discovery information.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHY layer, and a ProSe Protocol layer being an upper layer. Permission for announcement and monitoring of discovery information is handled in the upper layer ProSe Protocol. Contents of discovery information are transparent to an access stratum (AS). The ProSe Protocol allows only valid discovery information to be transferred to the AS for announcement.

An MAC layer receives discovery information from the upper layer ProSe Protocol. An IP layer is not used for transmitting the discovery information. The MAC layer determines a resource used in order to announce the discovery information received from the upper layer. The MAC layer makes and sends a protocol data unit (MAC PDU) to a physical layer. An MAC header is not added.

There are two types of resource assignments for announcing the discovery information.

1. Type 1

The type 1 is a method assigned so that resources for announcing the discovery information are not terminal-specific and the base station provides resource pool configuration for announcing the discovery information to the terminals. The configuration may be included in a system information block (SIB) to be signaled in a broadcast scheme. Alternatively, the configuration may be included in a terminal specific RRC message to be provided. Alternatively, the configuration may be broadcast-signaled or terminal-specific signaled of a different layer from the RRC message.

The terminal selects a resource from an indicated resource pool to announce discovery information using the selected resource. The terminal may announce discovery information through a resource optionally selected during each discovery period.

2. Type 2

The type 2 is a method where resources for announcing the discovery information are terminal-specifically assigned. A terminal in a RRC_CONNECTED state may request a resource for announcing a discovery signal to the base station through a RRC signal. The base station may assign a resource for announcing a discovery signal as an RRC signal. A resource for monitoring the discovery signal in a configured resource pool may be assigned in terminals.

With respect to a terminal in an RRC_IDLE state, a base station may report a type 1 resource pool for announcing the discovery signal as an SIB. Terminals where ProSe direct discovery is allowed use a type 1 resource pool for announcing the discovery information in the RRC_IDLE state. Alternatively, the base station 2) reports that the base station supports the ProSe direct discovery through the SIB but may not provide the resource for announcing the discovery information. In this case, the terminal should enter the RRC_CONNECTED state for announcing the discovery information.

With respect to a terminal in an RRC_CONNECTED state, the base station may configure whether to use a type 1 resource pool or a type 2 resource pool for announcing the discovery information through a RRC signal.

FIG. 13 illustrates an embodiment of a ProSe direct discovery procedure.

Referring to FIG. 13, it is assumed in a terminal A and a terminal B that a ProSe-enabled application program is operated, and the terminal A and the terminal B are configured in a friend relationship to each other, that is, a relationship capable of allowing D2D communication with each other in the application program. Hereinafter, the terminal B may be expressed as a friend of the terminal A. For example, the application program may be a social networking program. 3GPP Layers correspond to functions of an application program for using a ProSe discovery service regulated according to 3GPP.

A ProSe direct discovery between the terminal A and the terminal B may perform a following procedure.

1. First, the terminal A performs regular application-Layer communication with an application server. The above communication is performed based on Application programming interface (API).

2. A ProSe enabled application program of the terminal A receives a list of application layer IDs having a friend relationship. The application layer ID may generally be in the form of a network access ID. For example, an application layer ID of the terminal A may have a form such as adam@example.com.

3. A terminal A requests private expression codes for a user and private expression codes for a friend of the user.

4. 3GPP layers transmit an expression code request to a ProSe server.

5. The ProSe server map application layer IDs provided from an operator or a third application server to private expression codes. For example, an application layer ID such as adam@example.com. The mapping may be performed based on parameters (e.g., mapping algorithms, key values, and the like) received from an application service of the network.

6. The ProSe server responds the obtained expression codes to the 3GPP layers. The 3GPP layers report that expression codes with respect to the requested application layer are successively received to the ProSe enabled application program. Further, a mapping table between the application layer IDs and the expression codes are generated.

7. The ProSe enabled application program requests the 3GPP layers to start the discovery procedure. That is, when one of friends is located close to the terminal A and direct communication may be performed, the ProSe enabled application program attempts the discovery. 3GPP layers announce a private expression code of the terminal A (that is, “GTER543$#2FSJ67DFSF” which is a private expression code of adam@example.com in the above example). In mapping of an application layer ID of a corresponding application program and the private expression code, the mapping relationship may be known by the previously received friends, and the mapping may be performed.

8. It is assumed that the terminal B is operating the same ProSe enabled application program as that of the terminal A, and the above steps 3 to 6 may be executed. 3GPP layers included in the terminal B may perform ProSe discovery.

9. When the terminal B receives the above announce from the terminal A, the terminal B determines whether the private expression code included in the announce is known by the terminal B or is mapped to an application layer ID. As illustrated in step 8, since the terminal B performs steps 3 to 6, the terminal B knows a private expression code with respect to the terminal A, mapping of the private expression code to the application layer ID, and which is a corresponding application program. Accordingly, the terminal B may discover the terminal B from the announce of the terminal A. The 3GPP layers in the terminal B announces that adam@example.com is discovered to the ProSe enable application program.

FIG. 13 illustrates a discovery procedure by taking into consideration the terminals A and B, the ProSe server, and the application server. Only an operation side between the terminals A and B is described. The terminal A transmits a signal called the announce (the procedure may refer to announcement), and the terminal B receives the announce to discover the terminal A. That is, a discovery procedure of FIG. 13 in an operation directly related to another terminal among operations performed by each terminal may refer to a single step discovery procedure may refer to a single step discovery procedure in a side of one step.

FIG. 14 illustrates another embodiment of a ProSe direct discovery procedure.

In FIG. 14, it is assumed that the terminal 1 to the terminal 4 may be included in a specific group communication system enablers (GCSE) group. It is assumed that the terminal 1 is a discoverer and terminals 2, 3, and 4 are a discoveree. A terminal 5 is a terminal regardless of a discovery procedure.

The terminal 1 and the terminals 2 to 4 may perform a following operation in a discovery procedure.

First, the terminal 1 broadcasts a targeted discovery request message (hereinafter referred to ‘discovery request message’ or ‘M1’) in order to discover whether an optional terminal included in the GCSE group is located around the terminal 1. The targeted discovery request message may include a unique application program group ID or a layer-2 group ID of the specific GCSE group. Further, the targeted discovery request message may include a unique ID of the terminal 1, that is, an application program private ID. The targeted discovery request message may be received by the terminals.

The terminal 5 transmits no response messages. The terminals 2, 3, and 4 included in the GCSE group transmit a targeted discovery response message (hereinafter referred to as a discovery response message or M2) as a response to the targeted discovery request message. The targeted discovery response message may include a unique application program private ID of a terminal transmitting the message.

An operation of terminals in a ProSe discovery procedure illustrated in FIG. 14 will be described. A discoverer (UE 1) transmits the targeted discovery request message, and receives a targeted discovery response message being a response thereto. In addition, if a discoveree (e.g., UE 2) receives the targeted discovery request message, the discoveree transmits a targeted discovery response message as a response thereto. Accordingly, each terminal performs an operation a second step. In the above side, a ProSe discovery procedure of FIG. 14 may refer to a discovery procedure.

In addition to the discovery procedure illustrated in FIG. 14, if the terminal 1 (discoverer) transmits a discovery confirm message (hereinafter may refer to M3) as a response to the targeted discovery response message, this may refer to a third step discovery procedure.

Hereinafter, the present invention will be described.

When a UE is positioned outside of a network coverage (cell coverage), the UE may perform the D2D operation using a preconfigured resource. That is, when the UE determines that the UE itself is positioned outside of the network coverage, the UE may perform the D2D operation such as the D2D communication with another UE and the D2D discovery using the preconfigured resource pool. On the other hand, when the UE is positioned in the network coverage, it is the rule that the UE performs the D2D operation using the resource pool controlled by the network. Particularly, since the transmission of D2D signal may influence on another UE, the transmission of D2D signal may be performed under the control of the network. That is, when the UE is positioned in the network coverage, the UE may perform the D2D operation using the resource pool that is signaled by the network.

In the D2D operation, it may be required to accurately regulate when and under what condition a UE should be controlled by a network. For example, in the D2D operation, it may be required to accurately regulate when the resource pool should be applied, which is signaled by the network instead of the preconfigured resource that is used outside of the network coverage.

FIG. 15 exemplifies the case to which the present invention may be applied.

Referring to FIG. 15, a network coverage may be variously classified into, such as a first coverage 151 a second coverage 152 and a third coverage 153. The first coverage 151 is the coverage in which a stable connection is available between a UE and a network. The second coverage 152 may be the coverage in which the UE may receive the synchronization signal and the system information from the network, but the transmission power is low for the UE to transmit the uplink signal to the network. The third coverage 153 may be the coverage in which the UE may detect the synchronization signal only. The UE positioned outside of the third coverage 153 is unable to detect any signal from the network.

The UE may perform the D2D operation using the preconfigured resource outside of the third coverage 153.

It is assumed the case that the UE is positioned inside of the third coverage 153 and outside of the second coverage 152. In this case, the UE may receive the synchronization signal from the network, but may not receive the system information. Accordingly, it may be undesirable to change/switch the resource pool that is applied to the D2D operation for the UE since it is hard to determine that the UE is substantially within the network coverage, but rather, the UE may move outside of the third coverage 153 again. It may be required to change/switch the resource pool that is applied to the D2D operation so far as the UE moves into the second coverage 152 or the first coverage 151.

Meanwhile, it is preferable that the UE is provided with a different service according to the case that the UE is camped on or not camped on the suitable cell.

As such, it may be required to control the resource pool that the network is signaling by finely classifying it as the region/condition/state, and so on that the UE should apply to. For this control, the present invention proposes when (or in what condition) a UE performs the D2D operation using the resource pool that the network is signaling.

According to the first method, in the case that the suitable cell is signaling the information that notifies the resource pool whenever the UE is camped on the suitable cell, the UE may perform the D2D operation using the resource pool that the suitable cell is signaling. When the UE is in the RRC idle state, in the case that the UE is in the Any Cell Selection state, the UE stops the use of the resource pool information that the latest serving cell is signaling.

When the UE is in the RRC idle state, in the case that the UE is in the Any Cell Selection state or in the Camped on Any Cell state, the UE stops the use of the resource pool information that the latest serving cell is signaling.

That is, in the case that the UE is in the RRC idle state, the UE uses the resource pool information that the serving cell is signaling only when the UE is in the Camped normally state.

The resource pool information that the UE uses only in the Camped normally state may represent the resource for the D2D transmission.

In the case that the D2D operation is the D2D direct communication, when the UE in the RRC idle state, which is positioned within the network coverage, tries to perform a transmission of the D2D direct communication, the UE determines that the UE may perform the D2D transmission only in the case that the serving cell of the UE is the suitable cell. Accordingly, in the case that the UE is in the state of being camped on an acceptable cell, that is, the camped on any cell state or the any cell selection state, the UE determines that the UE is unable to stop or start the D2D transmission.

In the case that the D2D operation is the D2D direct discovery, when the UE in the RRC idle state, which is positioned within the network coverage, tries to perform a D2D discovery announcement, the UE may announce the D2D discovery message only when the serving cell of the UE is the suitable cell. When the serving cell of the UE is not the suitable cell or the UE is in the any cell selection state, the UE is unable to stop or start the announcement of the D2D discovery message.

The resource pool information that the UE uses only in the camped normally state may be the resource for the D2D reception. In the case that the D2D operation is the D2D direct communication, when the UE in the RRC idle state, which is positioned within the network coverage, tries to perform a reception of the D2D direct communication, the UE determines that the UE may perform the D2D reception only when the serving cell of the UE is the suitable cell. Accordingly, in the case that the UE is in the state of being camped on an acceptable cell, that is, the camped on any cell state or the any cell selection state, the UE determines that the UE is unable to stop or start the D2D reception. Meanwhile, in the case that the D2D operation is the D2D direct discovery, when the UE in the RRC idle state, which is positioned within the network coverage, tries to perform a D2D discovery monitoring, considering that the D2D discovery service is a sort of the best-effort service that may be performed within the range of not influencing on the cellular communication, the limitation of performing the D2D discovery monitoring depending on the state of UE based on the serving cell camping situation of the UE may be unnecessary. That is, exceptionally, in the case of the D2D discovery monitoring, the UE may perform the D2D discovery monitoring regardless of the UE state, which may be in camped normally state, camped on any cell state or any cell selection state.

When the UE is performing the D2D operation in a serving frequency, it is preferred to determine whether the D2D operation is available depending on whether the UE is camped on the serving cell or which serving cell the UE is camped on between the suitable cell and an arbitrary cell.

Instead, when the frequency on which the UE is tries to perform the D2D operation and the serving frequency are different, it may not proper to determine whether to perform the D2D operation depending on the state of UE based on the serving cell camping situation of the UE. In this case, it is available that the UE selects a cell for the D2D operation of the UE on the frequency on which the UE tries to perform the D2D operation and determines whether to perform the D2D operation on the frequency depending on the state of the selected cell or whether to select a cell. That is, it is preferable that the UE determines whether to perform the D2D operation depending on the state of the cell that the UE selects for the D2D operation, that is, the D2D operation cell, not the serving cell of the UE. For example, the scenario may be considered that the serving cell of the UE is cell #1 on frequency #1 but the D2D operation of the corresponding UE is performed in cell #2 on frequency #2. In such a scenario, the UE may determine whether to perform the D2D operation by determining whether cell #2, instead of cell #1, is the suitable cell (or the camped normally state) in the aspect of the UE. When the UE determines whether a specific cell is in the camped normally state for determining whether to perform the D2D operation, the UE determines whether the cell is belonged to a separate PLMN in which the D2D operation is allowed. In the case that the cell is belonged to the separate PLMN, the UE may determine that the cell satisfies the suitable cell condition in the aspect of the PLMN. When the separate PLMN list in which the D2D operation is allowed is configured to the UE, the PLMN list in which the D2D discovery operation is allowed and the PLMN list in which the D2D communication operation is allowed may be separately configured to the UE. In addition, the information representing whether the D2D discovery operation or the D2D communication operation is the PLMN in which the reception of the D2D signal is allowed or the PLMN in which the transmission of the D2D signal is allowed may be configured to the UE.

FIG. 16 exemplifies a D2D operation of UE according to the first method.

Referring to FIG. 16, a UE in the RRC idle state determines whether its serving cell is the suitable cell (i.e., in the Camped Normally state) (step, S210).

The camped normally state is referred to as the state in which a UE is camped on the suitable cell. In the camped normally state, the UE may select and monitor the paging channel according to the information given through the system information or perform the evaluation procedure for the cell reselection.

The UE in the RRC idle state determines whether the serving cell provides the resource pool information in the case that its serving cell is the suitable cell, that is, in the Camped Normally state (step, S220).

The resource pool information may be provided through the system information provided by the serving cell. The table below is an example of the system information including the resource pool information provided by the serving cell.

TABLE 2 -- ASN1START SystemInformationBlockType19-r12 ::= SEQUENCE {     discConfig-r12           SEQUENCE {     discRxPool-r12    SL-DiscRxPoolList-r12,     discTxPoolCommon-r12    SL-DiscTxPoolList-r12        OPTIONAL,  -- Need OR     discTxPowerInfo-r12    SL-DiscTxPowerInfoList-r12     OPTIONAL,  -- Cond Tx     discSyncConfig-r12        SL-SyncConfigList-r12        OPTIONAL  -- Need OR    }        OPTIONAL,  -- Need OR     discInterFreqList-r12 SL-CarrierFreqInfoList-r12     OPTIONAL,  -- Need OR     lateNonCriticalExtension OCTET STRING        OPTIONAL,    ... } SL-CarrierFreqInfoList-r12 ::= SEQUENCE (SIZE (1..maxFreq)) OF SL- CarrierFreqInfo-r12 SL-CarrierFreqInfo-r12::= SEQUENCE {     carrierFreq-r12        ARFCN-ValueEUTRA-r9,     plmn-IdentityList-r12    PLMN-IdentityList4-r12        OPTIONAL  -- Need OP } PLMN-IdentityList4-r12 ::= SEQUENCE (SIZE (1..maxPLMN-r11)) OF   PLMN- IdentityInfo2-r12 PLMN-IdentityInfo2-r12 ::= CHOICE {     plmn-Index-r9    INTEGER (1..maxPLMN-r11),     plmnIdentity-r12    PLMN-Identity } -- ASN1STOP

In the table above, ‘discInterFreqList’ indicates the neighbor frequencies in which the discovery announcement is supported. ‘discRxPool’ indicates the resources in which the reception of a discovery signal (e.g., the discovery announcement) is allowed in the RRC idle state or the RRC connected state. ‘discSyncConfig’ indicates the configuration in which the UE is allowed to transmit or receive the synchronization information. ‘discTxPoolCommon’ indicates the resources (resource pools) in which the UE is allowed to transmit the discovery signal (e.g., the discovery announcement) during the RRC idle state. ‘discTxPoolCommon’ may be an example of the resource pool information. ‘plmn-IdentityList’ are lists of the PLMN IDs. ‘plmn-Index’ is an index that corresponds to the entry in the ‘plmn-IdentityList’ field of SIB 1 (systeminformationblock type 1).

In the case that the serving cell of the UE in the RRC idle state is the suitable cell and the serving cell provides with the resource pool information, the UE transmits the D2D signal within the resource indicated by the resource pool information (step, S230).

For example, in the case that the serving cell of the UE in the RRC idle state is the suitable cell and ‘discTxPoolCommon’ is included in the system information provided by the serving cell, after selecting the resource pool among the resource pools indicated by the ‘discTxPoolCommon’, the UE may transmit the discovery announcement using the selected resource pool.

According to the second method, in the case that the suitable cell is signaling the information that notifies the resource pool whenever a UE is camped on a cell, the UE may perform the D2D operation using the resource pool that the suitable cell is signaling. When the UE is in the RRC idle state, in the case that the UE is in the Any Cell Selection state, the UE stops the use of the resource pool information that the latest serving cell is signaling.

According to the third method, while a UE is camped on a cell, in the case that the measurement result (e.g., RSRP) of the signal received from the cell is greater than a specific threshold value, the UE performs the D2D operation based on the information notifying the resource pool that the cell is signaling. That is, in comparison with the first method, the third method has the difference that the UE determines whether to use the information notifying the resource pool that the cell is signaling depending on the measurement result of the signal received from the cell.

The first method and the second method have the advantage that the operation is simple and the implementation is easy. The third method has the advantage that the region in which the D2D operation is controlled by the network may be more subdivided. In the present invention, for the flexibility of the network policy, all of the methods described above may be used.

When the first method or the third method is applied, a UE may be operated as follows.

While the UE is camped on a cell, the cell may broadcast the threshold value of signal strength and the resource pool information. In this case, the UE may compare the measurement result (e.g., RSRP) of the signal (e.g., reference signal) received from the cell with the threshold value. So far as the measurement result is greater than the threshold value, the UE may perform the D2D operation using the resource pool information that the cell is signaling.

Otherwise, while the UE is camped on a cell, the cell may broadcast the resource pool information only, and may not broadcast the threshold value of signal strength. In this case, the UE may perform the D2D operation using the resource pool information that the cell is signaling.

Or, while the UE is camped on a cell, the cell may not broadcast the resource pool information nor the threshold value of signal strength. In this case, the UE is unable to use the radio resource for the D2D operation in the frequency on which the cell is present. For example, the UE is preconfigured with the radio resource that may be used for the D2D operation outside of the cell coverage. However, the UE is unable to use the preconfigured resource after the UE is camped on a specific cell, and it is the rule that the UE should perform the D2D operation by the control of a specific cell (i.e., a network). Accordingly, the UE may not perform the D2D operation on the frequency of the specific cell unless the specific cell provides the resource for the D2D operation.

Hereinafter, the structure of the resource pool information that the serving cell is signaling will be described.

First, the points that should be considered when configuring the resource pool information will be described in the aspect of the transmission resource pool and the reception resource pool, and then, the detailed structure of the resource pool information will be exemplified.

<D2D Transmission Resource Information>

In the cellular communication, the transmission resource is controlled based on a cell. A UE is required to be controlled by a serving cell.

It is preferred to apply the rule in the same manner to the transmission of D2D signal. Accordingly, the UE transmits the D2D signal using the resource information that corresponds to the serving cell.

The D2D resource information may be the resource indication information (e.g., D2D transmission pool) that is similar to the time/frequency information in which the D2D resource is positioned. The D2D resource information may be a physical layer parameter such as the synchronization information (e.g., synchronization signal ID or synchronization signal timing information) for the D2D reception or the scrambling code that is applied to the D2D transmission signal.

In response to the signaling of the resource information that the UE is going to use for the D2D transmission, a network provides the information of the D2D transmission resource pool that corresponds to the serving cell only, but does not notify the transmission resource pool of a neighbor cell to the UE through the D2D transmission resource.

The UE that is going to transmit the D2D signal in mode 1 is not required to know the set of mode 1 transmission resources, that is, the mode 1 resource pool. This is because the UE may transmit the D2D signal using the resource indicated by the network since the network schedules the mode 1 transmission resource.

However, a reception UE that is going to receive the D2D signal in mode 1 within the coverage of the serving cell is required to know the mode 1 transmission resource used by a transmission UE. Accordingly, it is required to notify the resource information including the mode 1 transmission resource of the transmission UE to the reception UE within the serving cell. In this case, the resource information including the mode 1 transmission resource of the transmission UE may be notified to the reception UE using the mode 1 transmission resource, or preferably, the mode 1 reception resource.

The information indicating the mode 2 transmission resource that may be used for the D2D signal transmission in mode 2 should be notified to the UE by the network.

When signaling the transmission resource pool corresponding to the serving cell, the network may separately perform signaling such as by the mode 1 transmission resource pool and the mode 2 transmission resource pool.

Otherwise, when signaling the transmission resource pool corresponding to the serving cell, only the mode 2 transmission resource pool is signaled as a part of the transmission resource pool corresponding to the serving cell, but the mode 1 transmission resource pool may be signaled as a part of the reception resource pool.

<D2D Reception Resource Information>

In receiving the D2D signal, a network is not required to notice the reception resource information for mode 1 and mode 2 separately. This is because there is no difference in the operation of a reception UE regardless of which mode the transmission UE operates between mode 1 and mode 2.

On this point, the reception resource information that the network is signaling may be commonly applied to mode 1 and mode 2.

A serving cell and a neighbor cell may configure different D2D resource information, respectively. In order for a UE within the serving cell to receive the D2D signal transmitted using the transmission resource of the neighbor cell, the UE is required to know the resource information of the neighbor cell. The D2D resource information may be the resource indication information such as time/frequency information on which the D2D resource is positioned. The D2D resource information may be a physical layer parameter such as the synchronization information (e.g., synchronization signal ID or synchronization signal timing information) for the D2D reception or the scrambling code that is applied to the D2D transmission signal.

In order to notify the D2D resource information of the neighbor cell, one of following two methods may be used.

1) A method of notifying the D2D resource information to a UE using the common reception resource pool, which is the union of the resource pools of neighbor cells.

For example, there are neighbor cells #1, #2 and #3, and the resource pools of the neighbor cells are #1, #2 and #3. Then, a network notifies a single common resource pool that corresponds to the union of resource pools #1, #2 and #3 of neighbor cells #1, #2 and #3 as the reception resource pool for the UE. In order to receive the D2D signal using the neighbor cell transmission resource, the UE monitors only the single reception resource pool.

2) A method of separately notifying the resource pool of each of the neighbor cells.

For example, there are neighbor cells #1, #2 and #3, and the resource pools of the neighbor cells are #1, #2 and #3. Then, a network notifies the list of the resource pool including resource pools #1, #2 and #3 of the neighbor cells #1, #2 and #3 as the reception resource pool for the UE. In order to receive the D2D signal using the neighbor cell transmission resource, the UE should monitor the reception resource pool that corresponds to each of the neighbor cells.

3) A method of notifying the D2D resource information to a UE using the common reception resource pool, which is the union of the resource pools of neighbor cells, but notifying the physical layer parameter for each cell.

For example, there are neighbor cells #1, #2 and #3, and the resource pools of the neighbor cells are #1, #2 and #3. Then, a network notifies a single common resource pool that corresponds to the union of resource pools #1, #2 and #3 of neighbor cells #1, #2 and #3 as the reception resource pool for the UE. In order to receive the D2D signal using the neighbor cell transmission resource, the UE monitors only the single reception resource pool. With this method, the network signals the physical layer parameter of the D2D resource used in each cell to the UE for each neighbor cell independent of the common resource pool. Accordingly, in order to receive the D2D signal which is transmitted using the D2D resource of the neighbor cell, the UE performs the synchronization/de-scrambling by applying the physical layer parameter of each of the neighbor cells.

In the aspect of resource pool, when a UE knows all of the resource pools of the neighbor cells, the reception resource pool which is finally represented by the two methods becomes identical.

However, in order for the reception UE positioned in the serving cell to receive the D2D signal that the transmission UE positioned in the neighbor cell transmits, the reception UE is required to know the physical layer parameters for the neighbor cell, for example, cell-specific parameters such as a scrambling code, synchronization signal, and so on. In this point of view, the method of separately notifying the resource pool of each neighbor cell to the UE may be preferable.

In the case that different neighbor cells share the same physical layer parameters, the neighbor cells may be grouped. For the grouped neighbor cells, the method may be used for notifying the union of the resource pools of the neighbor cells to the UE as a single reception resource pool.

Meanwhile, the information representing the reception resource pools of the neighbor cells may be provided in the unit of cell. For example, a network may signal the list of the reception resource pools, and each of the resource pools in the list may be the resource pool of the corresponding neighbor cell.

The resource pool that corresponds to a serving cell may be the union of the mode 1 transmission resource pool and the mode 2 transmission resource pool of the serving cell. In this case, the network is not required to signal the resource pool corresponding to the serving cell independent of the mode 1 transmission resource pool and the mode 2 transmission resource pool.

The UE may configure the reception resource pool that corresponds to the serving cell, that is, the UE may configure the reception resource pool from the transmission resource pool. For example, the UE may configure the union of the mode 1 transmission resource pool and the mode 2 transmission resource pool as the reception resource pool that corresponds to the serving cell.

The table below exemplifies the structure of resource pool information that a serving cell is signaling.

TABLE 3 Entry No. Contents Information elements (IEs) Note 1 TX resource pool RscPool_mode1_tx_s, TX resource pool for mode 1 (of serving cell), RscPool_mode2_tx_s and TX resource pool for and parameters mode 2 PHY parameters Parameters of physical layer may be provided. 2 RX resource pool Absent UE may regard the union of (of serving cell), ‘RscPool_mode1_tx_s’ and and parameters ‘RscPool_mode2_tx_s’ as the RX resource pool of serving cell. PHY parameters Parameters of physical layer may be provided. 3 RX resource pool RscPool_mode1 + 2_rx_n1 Union of RX resources of of neighbor cell modes 1 and 2 for neighbor #1, and cell #1 parameters PHY parameters ID, synchronization information, etc. of neighbor cell #1 may be provided. 4 RX resource pool RscPool_mode1 + 2_rx_n2 Union of RX resources of of neighbor cell modes 1 and 2 for neighbor #2, and cell #2 parameters PHY parameters ID, synchronization information, etc. of neighbor cell #2 may be provided. . . . RX resource pool RscPool_mode1 + 2_rx_n . . . Union of RX resources of of neighbor cell modes 1 and 2 for neighbor #n, and cell #n parameters PHY parameters ID, synchronization information, etc. of neighbor cell #n may be provided.

The table below is another example of the structure of resource pool information that a serving cell is signaling.

TABLE 4 Entry No. Contents Information elements (IEs) Note 1 TX resource pool RscPool_mode2_tx_s TX resource pool for mode 2 (of serving cell), PHY parameters Parameters of physical layer and parameters may be provided. 2 RX resource pool RscPool_mode1_tx_s TX resource pool for mode 1 (of serving cell), UE may regard the union of and parameters ‘RscPool_mode1_tx_s’ and ‘RscPool_mode2_tx_s’ as the RX resource pool of serving cell. PHY parameters Parameters of physical layer may be provided. 3 RX resource pool RscPool_mode1 + 2_rx_n1 Union of RX resources of of neighbor cell modes 1 and 2 for neighbor #1, and cell #1 parameters PHY parameters ID, synchronization information, etc. of neighbor cell #1 may be provided. 4 RX resource pool RscPool_mode1 + 2_rx_n2 Union of RX resources of of neighbor cell modes 1 and 2 for neighbor #2, and cell #2 parameters PHY parameters ID, synchronization information, etc. of neighbor cell #2 may be provided. . . . RX resource pool RscPool_mode1 + 2_rx_n . . . Union of RX resources of of neighbor cell modes 1 and 2 for neighbor #n, and cell #n parameters PHY parameters ID, synchronization information, etc. of neighbor cell #n may be provided.

Meanwhile, a network and a UE exchange the D2D reception support information request and the D2D reception support information in response to the request, thereby more efficiently applying the resource pool information.

FIG. 17 illustrates a D2D operation method of UE according to an embodiment of the present invention.

Referring to FIG. 17, a UE receives the resource pool information from a network (step, S401).

The structure and configuration of the resource pool information have been described in Tables 3 and 4. The resource pool information may notify the transmission resource pool of a serving cell and the reception resource pool of at least one neighbor cell. That is, the resource pool information may indicate a plurality of resource pools that the UE should monitor.

The UE requests the D2D reception support information to the network (step, S402). In other words, the UE transmits the D2D reception support information to the network.

The UE may notify that the UE is going to receive the D2D signal (D2D message) to the network. The UE may notify it by transmitting a separate RRC message to the network. As such, when the UE notifies that the UE is going to receive the D2D signal, the UE may request the D2D reception support information. Otherwise, the UE may request the D2D reception support information through a separate procedure.

When the UE requests the D2D reception support information to the network, the UE may notify the information on the D2D signal that the UE is going to receive to the network.

For example, the UE may notify the transmission range of the D2D signal that the UE is going to receive to the network. As an example, the UE may notify that the UE is going to receive the D2D signal transmitted within 500 m range to the network. Otherwise, the UE may notify the transmission group of the D2D signal that the UE is going to receive to the network. For example, the UE may notify the transmission group identifier (Group ID) of the D2D signal that the UE is going to receive to the network. Or, the UE may notify the transmission UE of the D2D signal that the UE is going to receive to the network. As an example, the UE may notify the transmission UE identifier (UE ID) of the D2D signal that the UE is going to receive to the network.

Meanwhile, in order for the network to distinguish from which UE the network receives the D2D reception support information, the UE may include or mask an identifier (or identification information) of its own in the D2D reception support information request when requesting the D2D reception support information.

When the network receives the D2D reception support information request from the UE, the network transmits the D2D reception support information to the UE (step, S403).

The D2D reception support information may include the information required when the UE monitors the D2D signal. Based on the information on the D2D signal that the UE is going to receive, the D2D reception support information may include the corresponding information. That is, the D2D reception support information may include the information that may decrease the range of resource pool that the UE should monitor among a plurality of resource pools.

For example, the D2D reception support information may notify one or more reference cells that the UE should monitor for receiving the D2D signal. The reference cell may be associated with at least one reception resource pool. When the UE receives the reference cell information through the D2D reception support information, the UE may monitor the resource pool that is associated with the reference cell.

Or, the D2D reception support information may indicate one or more resource pools that the UE should monitor for receiving the D2D signal. The resource pool identified to the UE may be distinguished through a resource pool configuration or the identifier indicating a specific resource pool among a plurality of resource pools that is pre-signaled to the UE. The UE may perform the monitoring for the D2D signal reception using the indicated resource pool. That is, the D2D reception support information may indicate a specific resource pool among a plurality of resource pools, and through this, the range of resource pool that the UE should monitor may be decreased.

The UE performs the D2D signal reception (monitoring) using the reception resource pool that is indicated by the D2D reception support information (step, S404).

For example, it is assumed that the resource pool information for a serving cell and three neighbor cells (neighbor cells #1, #2 and #3) is provided with the resource pool information for each cell. And it is assumed that a specific UE is going to receive the D2D signal from the UEs that have a specific group ID only, but the UE does not know the current positions of the UEs that have the specific group ID.

In the case that there is no D2D reception support information, the specific UE should monitor all of the unions of the resource pools for each of the serving cell and three neighbor cells in order to receive the D2D signal that is transmitted by the UEs that have the specific group ID transmit.

On the other hand, the UE may notify the specific group ID to the network when requesting the D2D reception support information. And, the network may notify that the UEs are positioned in a specific neighbor cell (e.g., neighbor cell #2) by identifying the position of the UEs that have the specific group ID. Then, the specific UE may monitor the resource pool of neighbor cell #2 only.

FIG. 18 illustrates a D2D operation method of UE according to an embodiment of the present invention.

Referring to FIG. 18, a UE 2 transmits the D2D discovery signal to a UE 1 (step, S501). At the moment, the UE 2 may also transmit the ID of the UE 2 together.

The UE 1 receives the resource pool information from a network (step, S502).

The UE 1 requests the D2D reception support information to the network (step, S503). The D2D reception support information request may include the ID of the UE 2.

After receiving the D2D reception support information request from the UE 1, the network transmits the D2D reception support information to the UE 1 (step, S504). The D2D reception support information may include the reception resource pool information of the UE 2.

The UE 1 performs the D2D communication signal reception (monitoring) using the reception resource pool indicated by the D2D reception support information (step, S505).

For example, the UE 1 may receive the D2D discovery signal transmitted by the UE 2, but specific reception pool information may be required to receive the D2D communication signal transmitted by the UE 2. In such a case, the UE 1 may notify the ID of the UE 2 included in the D2D discovery signal transmitted by the UE 2 to an eNB (network). The eNB may indicate the resource pool that is required to receive the D2D communication signal of the UE 2 to the UE 1. The UE 1 performs the D2D communication signal monitoring using the indicated resource pool.

FIG. 19 is a block diagram illustrating a UE in which the embodiments of the present invention are implemented.

Referring to FIG. 19, a UE 1100 l includes a processor 1110, a memory 1120 and a radio frequency (RF) unit 1130. The processor 1110 implements the proposed functions, processes and/or methods. For example, the processor 1110 transmits the D2D reception support information request to the network, and receives the D2D reception support information in response to the request from the network. The D2D reception support information includes the information that may decrease the range of resource pool that the UE should monitor among a plurality of resource pools.

The RF unit 1130 is connected with the processor 1110, and transmits and receives radio signals.

The processor 1130 may include an application-specific integrated circuit (ASIC), a separate chipset, a logic circuit, and/or a data processing unit. The memory may include a read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, storage medium, and/or other equivalent storage devices. The RF unit may include a base-band circuit for processing a radio signal. When the embodiment of the present invention is implemented in software, the aforementioned methods can be implemented with a module (i.e., process, function, etc.) for performing the aforementioned functions. The module may be stored in the memory and may be performed by the processor. The memory may be located inside or outside the processor, and may be coupled to the processor by using various well-known means. 

What is claimed is:
 1. A method for a device-to-device (D2D) operation performed by a user equipment (UE) in a wireless communication system, comprising: receiving, from a network, D2D reception support information; and receiving a D2D signal using a reception resource pool indicated by the D2D reception support information, wherein the D2D reception support information includes information that is available to decrease a range of resource pool that the UE should monitor among a plurality of resource pools.
 2. The method of claim 1, further comprising transmitting a D2D reception support information request to the network.
 3. The method of claim 2, wherein the request includes an identity (ID) or a group ID of another UE that transmits the D2D signal that the UE is going to receive.
 4. The method of claim 2, wherein the request includes identification information that is available to identify the UE.
 5. The method of claim 1, wherein the D2D reception support information notifies a reference cell that the UE should monitor.
 6. The method of claim 1, wherein the D2D reception support information indicates a specific resource pool among the plurality of resource pools.
 7. The method of claim 1, further comprising receiving resource pool information, wherein the resource pool information indicates the plurality of resource pools.
 8. A user equipment (UE) for performing a device-to-device (D2D) operation in a wireless communication system, comprising: a radio frequency (RF) unit that transmits or receives a radio signal; and a processor operatively connected to the RF unit, wherein the processor: receives D2D reception support information from a network; and receives a D2D signal using a reception resource pool indicated by the D2D reception support information, wherein the D2D reception support information includes information that is available to decrease a range of resource pool that the UE should monitor among a plurality of resource pools.
 9. The UE of claim 8, wherein the processor transmits a D2D reception support information request to the network.
 10. The UE of claim 9, wherein the request includes an identity (ID) or a group ID of another UE that transmits the D2D signal that the UE is going to receive.
 11. The UE of claim 9, wherein the request includes identification information that is available to identify the UE.
 12. The UE of claim 8, wherein the D2D reception support information notifies a reference cell that the UE should monitor.
 13. The UE of claim 8, wherein the D2D reception support information indicates a specific resource pool among the plurality of resource pools.
 14. The UE of claim 8, wherein the processor receives resource pool information, wherein the resource pool information indicates the plurality of resource pools. 